Qubit protection using interference

The Problem

Fast, high-fidelity measurement requires readout resonators with large bandwidths (κ/2π ≫ 10 MHz). The downside is that these resonators open a decay channel for the qubit, leading to rapid decoherence through Purcell loss. Standard fixes rely on bandpass filters, but those add circuit components and consume chip area.

The Approach

I developed an interferometric Purcell filter that suppresses spontaneous emission through destructive interference—no extra components required. A λ/4 resonator couples to the feedline at two points: capacitively at its open end and inductively at its shorted end. By setting the separation to $λ_qubit/4$, the couplings interfere to block emission at the qubit frequency.

Results & Impact

• >100× Purcell suppression across a 400 MHz bandwidth
• Purcell-limited qubit lifetime is 16+ ms at the filter notch
• No additional circuit components needed
• Seamlessly compatible with existing readout architectures

Why This Matters

This approach provides qubit protection with no extra filters and no architectural overhaul. It improves lifetimes while keeping hardware efficient, improving the scaling prospects of quantum computers.